//forÇ¶Ì×if,Çó1µ½¸ø¶¨ÊýNÒÔÄÚËùÓÐÆæÊýµÄºÍ
int main()

{
  int i,N,sum = 0;
  scanf(N);
  for(i=1;i<=N;i=i+1)
{

     if(i%2 == 1)
  sum = sum+i;
    }

  printf(sum);
}